Used for environmental protection are honeycomb filters for removing carbon-based particulate matter from exhaust gases discharged from diesel engines. The honeycomb filter is alternately sealed on both end surfaces on the exhaust-gas-inlet and exit sides. FIG. 4 shows an example of conventional ceramic honeycomb filters having such structure. This ceramic honeycomb filter 50 is constituted by a porous ceramic honeycomb structure 11 comprising porous cell walls 3 for forming pluralities of flow paths 2a, 2b and a peripheral portion 1 a enclosing the porous cell walls 3, plugs 55 for sealing flow paths 2b in a checkerboard pattern in an end portion of the porous ceramic honeycomb structure 11, and plugs 56 for sealing flow paths 2a in the other end portion of the porous ceramic honeycomb structure 11 in such a checkerboard pattern that they do not overlap the plugs 55. The exhaust gas containing particulate matter enters the flow paths 2a through the inlet-side opening end 57, passes through cell walls 3 and adjacent flow paths 2b, and goes out of the exit-side end surface 58. In this case, the particulate matter in the exhaust gas is captured in pores (not shown) in the cell walls 3. As the particulate matter continues captured by the ceramic honeycomb filter 50, pores in the cell walls 3 are clogged, resulting in drastic decrease in a particulate-matter-capturing function, and increase in pressure loss that reduces engine power. The accumulated particulate matter (PM) is combusted by an electric heater, a burner, microwaves, etc., to regenerate the ceramic honeycomb filter 50. The oxidation of the accumulated particulate matter is accelerated by a catalyst carried on the ceramic honeycomb filter 50, to regenerate the ceramic honeycomb filter 50.
Under usual operation conditions of diesel engines, however, high exhaust gas temperatures to combust PM cannot be achieved. Accordingly, investigation has been being conducted on technologies for regenerating the honeycomb filter 50 by accelerating the oxidation of PM by a catalyst carried on the honeycomb filter 50. For instance, a honeycomb filter integrally having a catalyst comprising a platinum-group metal and a rare earth oxide such as cerium oxide carried on alumina, a material having a high specific surface area, has been put into practical use. Using such catalyst-carrying honeycomb filter, a combustion reaction can be accelerated by the catalyst to remove the accumulated PM.
In the honeycomb filter 50 having a conventional structure shown in FIG. 4, however, PM is easily attached to an inlet-side end surface 57 having low catalyst activity, particularly to end surfaces of the inlet-side plugs 55, clogging the inlet-side flow paths 2a of the filter, and thus increasing pressure loss. To make the honeycomb filter easily regeneratable with improved cleaning function, a honeycomb filter 10 having plugs 5 in flow paths at positions separate from the inlet-side end surface 7 as shown in FIG. 1 has been proposed.
JP 3-232511 A discloses a method for plugging a ceramic honeycomb structure on both inlet- and exit-end surfaces in a checkerboard pattern, comprising the steps of attaching a porous sheet to one end surface of the ceramic honeycomb structure, attaching a shield sheet to the other end surface, introducing a ceramic plug material through apertures of the porous sheet into flow paths to positions near the other end surface, and solidifying the ceramic plug material to plug the end portions of the flow paths. However, this method produces a honeycomb filter 50 having the conventional structure shown in FIG. 4, failing to form plugs in the flow paths deep from the end surface.
JP 2004-536692 A discloses a method for forming plugs in an end portion of a flow path by charging ceramic powder slurry from an upper end to a lower end portion. However, this method cannot form plugs in flow paths deep from the end surface, and ceramic powder attached to intermediate portions of the flow paths clogs pores in cell walls, resulting in large pressure loss.
Methods for forming plugs in a honeycomb filter at positions separate from the inlet-side end surface are disclosed by JP 3-68210 B, JP 6-33739 A and JP 2004-19498 A.
The method of JP 3-68210 B comprises, as shown in FIG. 5(a), the steps of sealing ends of flow paths needing no plug in a porous ceramic honeycomb structure 61 with a wax 66, immersing the honeycomb structure 61 in a plug-forming slurry 69 with the inlet-side end 67 downward, and charging the slurry 69 into the flow paths 62 not sealed by the wax. Because the honeycomb structure 61 per se absorbs water, the slurry entering the flow paths 62 is deprived of water more in an upper portion than in a lower portion by the cell walls 64, thereby forming a solidified portion 65a. As shown in FIG. 5(b), when the honeycomb structure 61 is turned upside down, water is removed from the slurry remaining on the solidified portion 65a, the slurry shrinks during solidification. As a result, gaps are provided between the plugs 65 and the end surface 67 of the honeycomb structure. The position of the inlet-side plugs 65 is determined by the amount of slurry introduced into the flow paths 62. However, experiment has revealed that because solidification occurs to a relatively large extent in a lower portion of the slurry 69 charged into the flow paths 62, the solidified slurry remains in a nonnegligible amount on cell walls 64 between the end surface 67 and the solidified portion 65a when the honeycomb structure 61 is turned upside down, resulting in large pressure loss upstream of the inlet-side plugs 65.
JP 3-68210 B also discloses a method for forming inlet-side plugs integral with cell walls by embedding ceramic chips in flow paths of an extrusion-molded honeycomb structure, and sintering them. However, because it is difficult to make the expansion coefficient of the extrusion-molded honeycomb structure completely equal to that of the ceramic chips, gaps are generated between the ceramic chips and cell walls due to expansion and shrinkage by sintering, resulting in a small particulate-matter-capturing effect and insufficient bonding to the cell walls. It has thus been found that plugs are detached, and that the ceramic chips destroy the cell walls.
JP 6-33739 A discloses a method for producing a ceramic honeycomb filter comprising the steps of embedding a first paste containing cordierite powder in flow paths of a honeycomb green body in upstream-side end portions, charging a second paste containing cordierite powder and combustible powder from the side upstream of the first paste to push the first paste by a predetermined distance, and sintering them to remove the combustible powder, thereby forming plugs in the flow paths at upstream positions separate from the end surface. In this method, however, even after the combustible powder in the first paste filling portions upstream of the inlet-side plugs is burned out, the cordierite powder is attached to the cell walls by sintering to some extent, thereby partially clogging pores in the cell walls and thus accelerating the accumulation of particulate matter.
JP 2004-19498 A discloses a method for forming plugs in flow paths of a honeycomb structure deep by a predetermined distance from the upstream-side end surface, by charging an aqueous paste containing cordierite and an organic binder into a syringe having a long needle, and introducing the aqueous paste from the syringe into each flow path alternately with a predetermined depth from the upstream-side end surface. It has been found, however, that cell walls of the flow paths are destroyed by the contacting syringe needle in this method. Also, the paste is solidified in the syringe needle, causing clogging. Further, because the syringe needle is inserted into individual flow paths, an extremely long period of time is needed to form plugs particularly in a large ceramic honeycomb filter having an outer diameter exceeding 150 mm, which has 10000 flow paths or more.